Chlorine dioxide used as an aqueous solution is of considerable commercial interest and importance, mainly in the area of pulp bleaching but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes by which the chlorine dioxide can be efficiently produced.
The predominant chemical reaction involved in such processes is summarized by the formula EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O (1)
The chlorate ions are provided by alkali metal chlorate, preferably sodium chlorate, the chloride ions by alkali metal chloride, preferably sodium chloride, or by hydrogen chloride, and the hydrogen ions by mineral acids, normally sulfuric acid and/or hydrochloric acid.
There are also competing reactions which decrease the efficiency of the formation of chlorine dioxide from chlorate ions, the main reaction being represented by the formula EQU ClO.sub.3.sup.- +6H.sup.+ +5Cl.sup.- .fwdarw.3Cl.sub.2 +3H.sub.2 O (2)
The efficiency of commercial processes, such as the SVP process (SVP is a registered Trade Mark owned by KemaNord, Sweden) are normally above 90% and preferably above 95%, which means that the competing reaction represented by formula (2) is very much suppressed.
Processes for producing chlorine dioxide are set forth in e.g. U.S. Pat. Nos. 3,563,702 and 3,864,456, which are hereby incorporated by reference, comprising continuously feeding to a single vessel generator-evaporator-crystallizer alkali metal chlorate, an alkali metal chloride and mineral acid solutions in proportions sufficient to generate chlorine dioxide and chlorine, at a temperature of from about 50 to about 100 degrees centigrade, and an acidity of from about 2 to about 12 normal, with or without a catalyst, removing water by vacuum-induced evaporation at about 100-400 millimeters of mercury absolute, with concurrent withdrawal of chlorine dioxide and chlorine, crystallizing the salt of the mineral acid within the generator and withdrawing the crystals from the vessel.
In those reaction systems wherein the acid normality is maintained between about 2 and 4.8, the reaction may be carried out in the presence of a relatively small amount of a catalyst, such as those selected from the group consisting of vanadium pentoxide, silver ions, manganese ions, dichromate ions and arsenic ions.
As the reaction occurs within the generator in producing chlorine dioxide from sodium chlorate and sodium chloride, where sulfuric acid is employed as a mineral acid reactant, crystals of sodium sulfate are crystallized and withdrawn in the form of a slurry. The main reaction is shown by the formula EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +Na.sub.2 SO.sub.4 +H.sub.2 O (3)
Sodium sulfate is a valuable by-product useful in kraft pulping operation. It is used in the chemical recovery system in order to cover losses of sulfur and sodium.
In some cases, however, the requirement for sodium sulfate is greatly reduced or obviated. In certain kraft mill operations, the requirements for sodium sulfate may be reduced or varied. While the requirement for reduced quantities of sodium sulfate may vary, the requirement for the chlorine dioxide remains or even increases.
In order to reduce the quantities of sodium sulfate it has been suggested in U.S. Pat. No. 3,933,987 to substitute part of the sodium chloride with hydrochloric acid as a source for the reducing agent. Here, the hydrochloric acid serves the dual function as a reducing agent and as a portion of the mineral acid. By using the combination of sulfuric acid and hydrochloric acid the amount of precipitated sodium sulfate can be effectively controlled and reduced. When one mole of hydrochloric acid and half a mole of sulfuric acid are used, the amount of precipitated sodium sulfate is reduced by 50% compared to the mode when all chloride ions are added as sodium chloride, as can be seen by comparing formula (3) with the following formula: EQU NaClO.sub.3 +HCl+1/2H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +1/2Na.sub.2 SO.sub.4 +H.sub.2 O (4)
The main reactions for the chlorine dioxide production always also produce half a mole of chlorine per mole of chlorine dioxide, see formulas (1), (3) and (4). An additional small amount of chlorine is also produced by the side reaction according to formula (2). This chlorine product has formerly been used as such in the paper mills as a bleaching agent in aqueous solution, or it has been reacted with sodium hydroxide to form sodium hypochlorite and used as a bleaching agent. Today there is a tendency towards a more extensive chlorine dioxide bleaching and thus a decreasing need for chlorine and hypochlorite as bleaching agents. Another disadvantage with by-product chlorine is that the concentration of the chlorine solution is very low, normally 1-5 g/l. Because of the large amount of water, modern bleaching systems cannot use the by-product chlorine in the chlorination stage. This means that many pulp mills consider the chlorine as a by-product of limited value.
In order to make the chlorine dioxide process more attractive, it has been suggested that the amount of chlorine leaving the plant can be reduced by reacting the by-product chlorine with sulfur dioxide and producing a mixture of sulfuric acid and hydrochloric acid according to the formula EQU Cl.sub.2 +SO.sub.2 +2H.sub.2 O.fwdarw.2HCl+H.sub.2 SO.sub.4 ( 5)
The produced mixed acid can thereafter be used directly as acid feed to the chlorine dioxide reactor operating in a partial hydrochloric acid mode according to formula (4).
Such processes have been previously described in U.S. Pat. No. 3,347,628 and Swedish published patent application No. 8103892. In U.S. Pat. No. 4,086,329 a balanced process is described, where just sufficient sulfuric acid is produced according to formula (5) in order to get the exact need of sulfate ions to be combined with the sodium ions introduced to the chlorine dioxide generating system with the sodium chlorate.
The reaction of sulfur dioxide and chlorine obtained in the chlorine dioxide generating process is also disclosed in U.S. Pat. No. 3,393,036. Here the reaction is effected in an aqueous medium at a temperature below the boiling point of water. In order to accomplish this, the reaction is carried out in a cooled falling film tower having integral cooling passages or in a packed bed tower with a substantial portion of the produced mixed acid being recycled through an external heat exchanger. This patent shows the difficulties in controlling the exothermic reaction of chlorine and sulfur dioxide.
In the manufacturing of mixed acid from chlorine and sulfur dioxide by their reaction with water there are two main factors to consider: (i) The process is strongly exothermic with a .DELTA.H reaction around 80 kcal/mol and thus requires an effective cooling. (ii) The gaseous reactants must be transferred to the liquid phase and there react with each other and water, thus an effective mass transfer is necessary in order to obtain acceptable yields.
In the U.S. Pat. No. 4,393,036 an extensive cooling has been accomplished by either external or internal cooling with a cooling agent. This requires either recirculation through a heat exchanger by the aid of an external pump or a complicated apparatus with a manifold of internal cooling channels through which the cooling agent is circulated. In the later case there is a risk of leakage because of the corrosive mixed acid and the leakage can lead to contamination of the cooling water.